Programming Model and Protocols for Reconfigurable Distributed ...

20 CHAPTER 2. COMPONENT MODEL
its execution, a handler may trigger new events and mutate the component’s
local state. The Kompics execution model guarantees that the event
handlers of one component instance are mutually exclusive, i.e., they are
executed sequentially. This alleviates the need for synchronization between
different event handlers of the same component accessing the component’s
mutable state, which greatly simplifies their programming.
Code 2.5 illustrates an example event handler definition in Java. Upon
receiving a Message event, the handleMsg event handler increments a local
message counter and prints a message to the standard output console.
Code 2.5 A simple event handler
1 Handler handleMsg = new Handler() {
2 public void handle(Message message) {
3 messages++; // ← component-local state update
4 System.out.println("Received from " + message.source);
5 }
6 };
In diagrams, we use the hEvent graphical notation to represent an
event handler, where h is the handler’s name and Event is the type of events
accepted by the handler, e.g., Message.
2.1.5 Subscriptions
A subscription binds an event handler to a component port, enabling the
event handler to handle events that arrive at the component on that port. A
subscription is allowed only if the type of events accepted by the handler,
say E, is allowed to pass by the port’s type definition. In other words, if E
is the set of event types that the port allows to pass in the direction of the
event handler, then either E ∈ E, or E must be a subtype of a member of E.
Figure 2.3 illustrates the handleMsg event handler from our previous
example, being subscribed to the component’s required Network port. In
diagrams, we represent a subscription using the graphical notation.
In this example, the subscription of handleMsg to the Network port is
allowed because Message is in the positive set of Network. As a result of this
subscription, handleMsg will handle all events of type Message or of any
subtype of Message, received on this Network port.